Typically, fuel injection systems comprise a plurality of fuel electroinjectors, each provided with a metering servo valve comprising a control chamber supplied with pressurized fuel and provided with a fuel outlet normally closed by an open/close element via elastic urging means. The open/close element is operated to open the fuel outlet of the control chamber by an electric actuator acting in opposition to the elastic urging means to cause fuel to be injected. The fuel pressure in the control chamber acts on a control rod axially movable in the injector body, which control rod engages with a nebulizer needle axially mobile to open and close fuel injection holes in a nebulizer nozzle.
The fuel injection system further comprises an electronic control unit programmed to supply the electric actuators, for each fuel injection, with a corresponding electrical command. The time delay of the movement of the control rod with respect to the corresponding electrical command depends upon the pre-loading of the urging means that act on the open/close element of the metering servo valve, as well as upon the volume of the control chamber and upon the ratio between the sections of the fuel inlet and outlet thereof.
In order to improve engine performance, from EP 1657422 and EP 1795738 a fuel injection system is known in which, in predefined engine operating conditions (based on the engine speed, load, coolant temperature, etc.), the electronic control unit supplies, in a fuel injection phase and in the corresponding fuel combustion phase in an engine cylinder, at least a first electrical command of a predetermined time duration to perform a pilot fuel injection, and a subsequent electrical command of a time duration depending upon the engine operating conditions to perform a main fuel injection. The two electrical commands are separated in time by an electrical dwell time such that the main fuel injection starts without interruption with respect to the pilot fuel injection, i.e., the instantaneous fuel flow-rate during the fuel injection phase assumes a so-called “two-hump profile”.